(1) Field of the Invention:
The present invention relates to a process for firing cordierite-based ceramic honeycomb structural bodies. In particular, the invention relates to the firing process suitable for firing honeycomb structural catalyst carriers having high strength and low thermal expansivity and being adapted to be used for the purification of exhaust gases from automobiles.
(2) Related Art Statement:
Heretofore, it has been conventional practice to produce cordierite-based ceramic honeycomb structural bodies by the steps of forming a ceramic body through formulating and mixing a cordierite-forming raw ceramic material with a shaping aid or a pore-forming agent, producing a ceramic honeycomb-shaped body by extruding the resulting ceramic body, and firing the ceramic honeycomb-shaped body at a given temperature in a continuous kiln or a periodic kiln.
Among other characteristics, the ceramic honeycomb structural bodies to be used as catalyst carriers for the purification of exhaust gases from automobiles must have a high water-absorbing rate to improve catalyst-carrying ability and a low coefficient of thermal expansion to improve thermal shock resistance. In order to attain appropriate values of the product characteristics, it has been a conventional practice to control the maximum temperature and the holding time at the maximum temperature during firing.
Now, consider the case where the honeycomb structural body is produced by controlling the maximum temperature and the holding time as mentioned above. In order to increase the water-absorbing rate, since the porosity in honeycomb ribs needs to be increased, it is necessary to restrain the sintering by lowering the maximum temperature or by reducing the holding time during firing. On the other hand, in order to lower the coefficient of thermal expansion, since the body needs to be densified, it is necessary to promote sintering through raising the maximum temperature or prolonging the holding time during firing.
Therefore, both the high water-absorbing rate and the low coefficient of thermal expansion cannot be optimized only by controlling the maximum temperature and the holding time during firing. Further, since the characteristics of the product may largely change due to variation in properties of the raw materials, such as grain size or the average particle diameter, it is difficult stably to obtain various characteristics only by controlling the maximum temperature and the holding time.
On the other hand, Japanese patent application Laid-open No. 53-82,822 discloses a technique to attain a low coefficient of thermal expansion through the adjustment of the raw materials, in which the coefficient of thermal expansion can be lowered by increasing the heating rate in a temperature range of not less than 1,100.degree. C. However, this technique is insufficient from the standpoint of simultaneously attaining low thermal expansivity and high porosity.